This invention relates to means for ensuring that in the event of overheating caused, for example, by fire, an electronic control system for a gas turbine engine will fail in a way which safeguards the mechanical integrity of the engine.
In the unlikely event of a fire in or near a gas turbine engine, causing overheating of an electronic control system of the engine, it is important that the control system should fail in a safe, predictable way. For example, a failure which caused the control system to allow a significantly higher fuel flow rate to the engine than it actually required could lead to a catastrophic overspeed of the engine. Such an event is of particular concern in the case of an engine fitted with a FADEC (Full Authority Digital Electronic Control) system, where the integrity of the engine is totally dependent on the control system, so it is even more important that the system fails in a safe manner.
It is conventional to tackle this problem by simply arranging the system such that if overheating occurs, the control system is disconnected from its power supply, thereby shutting off the valves in the fuel flow regulator so that the engine is starved of fuel and runs down. In the past, the means of achieving the disconnection have been essentially mechanical in nature, such as bimetallic strips or fusible wires. Such means are becoming unsatisfactory because compared with other items of modern gas turbine technology, they have an unacceptably high failure rate in excess of three per million hours during normal engine running conditions. They also do not have very good temperature sensing accuracy characteristics.